IT is well known that animals and plants inhabiting freshwater have, as a general rule, a very wide distribution; yet each river system, with all the pools and lakes in connection with it, seems completely cut off from every other system of the same country. Still more complete is the separation between the freshwaters of distinct continents or of islands; nevertheless they often possess freshwater species in common. In my "Origin of Species"1 I have suggested various means of transportal; but as few facts on this head are positively known, the case given in the adjoined letter of a living Unio, which had caught one of the toes of a duck's foot

between its valves, and was secured in the act of being transported, seems to me well worth recording.

CHARLES DARWIN

DEAR SIR—The following case will, I think, prove of interest to you, as it corroborates your belief that freshwater shells are sometimes transplanted by the agency of aquatic birds.

In the sketch I have endeavoured to give you a correct idea of the way in which the shell was attached to the duck's foot.

It was given to me by Mr. H. L. Newcomb, who shot the bird, which was a blue-winged teal (Querquedula discors), while flying, near the Artichoke river at West Newbury, Mass., September 6, 1877. The shell, the common mussel, or clam (Unio complanatus), is a very abundant species, being found in nearly all the rivers and ponds of the Atlantic slope. How long the shell had been attached is only a matter of conjecture, but it had abraded the skin of the bird's toe, and left quite an impression. It was living when the bird was shot.

It would have undoubtedly been transplanted to some pond or river, perhaps miles from its original home, had the bird not been shot, and might then have propagated its kind.